[Tinycc-devel] __COUNTER__ isn't documented

[Temirkhan Myrzamadi]

The following code snippet:

```
__COUNTER__
__COUNTER__
__COUNTER__
```

Produces the following result:

```
$ tcc tcc.c -std=c99 -E
# 1 "tcc.c"
0
1
2
```

But `__COUNTER__` appear to be undocumented here:
https://bellard.org/tcc/tcc-doc.html.

Maybe it's better to document it explicitly?

```
$ tcc -v
tcc version 0.9.27 (x86_64 Linux)
```